Field of the Invention and Related Art Statement
The present invention relates to a method of recording information on an optical record medium, and more particularly to a method of recording information with the edge recording on an optical record medium such as magneto-optic record disk and write-once type optical disk and card.
Presently there are two known methods of recording information, e.g. channel bits on an optical record medium such as an optical record disk. One, the most commonly practiced today, is called the mark position recording method, where a write laser light beam is either turned on or turned off at the proper location and the written spots have always the same size. The marked spot within the detection window is a "1" bit, while the un-marked spot represents a "0" bit. Another technique is called the edge recording method, where the mark size or mark length measured along an information track on the optical record disk is intentionally varied by proper modulation of the writing laser beam. The transition which occurs at the boundary of the mark is a "1" bit, while the length of the mark represents the number of "0" bits between successive "1" bits.
The edge recording method has the potential to provide a significant gain G in recording density over the mark position recording method. The gain G is dependent on the code with which the data bits are translated into channel bits. It is not hard to show that EQU G=(d+1)/.alpha. (1)
where d is a code parameter out of (d, k, m, n, r) and specifies the minimum number of "0" bits between two nearest "1" bits. It should be noted that k specifies the maximum number of "0" bits between two nearest "1" bits. For example, in (1, 7) RLL (Run Length Limited) code, d is 1 and in (2, 7) RLL code, d is 2, .alpha. is a factor in mark position recording given by, EQU .alpha.=L.sub.t, .sub.min /L.sub.c ( 2)
where L.sub.t, .sub.min is the smallest mark length and L.sub.c is the unit channel bit length.
Let us use (2, 7) code as an example where d=2, and .alpha. is about 1.5, according to industry experience with (2, 7) code optical recording, therefore G is 2. This is an impressive gain if it can be realized.
The difficulties of the edge recording lie in several areas:
(a) It is difficult to write marks of long length with a single pulse. This is because the mark size will increase both along the information track direction and perpendicular to the track direction, and will become large enough to cause cross-talk problem with adjacent tracks if the intended mark length is significantly longer than the track pitch. It has been indicated by the computer simulation that the cross track dimension of the mark becomes 1.6 .mu.m when the mark length exceeds 2 .mu.m under reasonable media parameters and recording conditions. PA0 (b) The read domain or mark length (RDL) and the written domain or mark length (WDL) are different from each other because the read signal is a convolution of the optical spot with the written mark; the difference is dependent on the read spot size and on the mark length. Since the spot intensity has roughly a Gaussian-like distribution, with a Gaussian diameter of about 1.5 .mu.m for an optical head with an NA of 0.55, the convolution result, i.e. the read mark length RDL is quite different from the written mark length WDL. Applicant's simulation work indicates that when WDL is small, less than 1 .mu.m or so, the difference WDL-RDL is negative and is a strong function of WDL, while when WDL is larger than 1 .mu.m, RDL-WDL turns positive and becomes a constant when WDL is larger than 1.6 .mu.m. Since RDL is what counts in the detection process, it is necessary to translate the desired RDL into the proper WDL during the recording process. PA0 (c) In the edge recording method the mark length has to be very accurate, much more so than with the mark position recording method. In the latter method, the presence or absence of a mark is what counts. Since peak detection method is used to determine this, the detection is less prone to error even if the mark is somewhat larger than the window allocated to the particular channel bit. With edge recording method, if the minimum mark length changes by .+-.x percent, then the "1" bit window at either end of the mark has shrunk by y percent, where y is given by, EQU y=x .multidot.(d+1) (3)
Thus, if the mark length increases or decreases by 5%, the detection window loss is 15% in (2, 7) code and 10% in (1, 7) code, which is significant. Since the window size is small to start with for high density recording, such loss is unbearable from data detection point of view.
Factors that contribute to written mark length variation include media sensitivity variation from one disk to another, temperature variation in the drive and write power variation. These causes depend on external factors while the problem mentioned in (b) can be predicted a-priori.
In Japanese Patent Application Laid-Open Publication Kokai Hei No. 2-54423, there is described an improved edge recording method. In this known edge recording method, a mark having the shortest length is written by a single pulse, but marks having a longer length than the shortest length are recorded with the aid of a plurality of recording pulses. That is to say, during the writing of the longer marks, the writing laser beam is repeatedly turned on and off. By modulating the writing laser beam in the manner explained above, the heat given by the writing laser beam can be lost relatively quickly and residual heat becomes small, so that the enlargement of written marks is somewhat limited. However, the written marks having longer lengths are still prolonged. In the above-mentioned Japanese Publication it is also proposed that the power of the writing laser beam pulses be changed in dependence on the sensitivity of the optical disk. That is to say, the power of the writing laser beam pulses is controlled to such a level that the rear edge of the written mark becomes coincident with a desired position. However, it has been experimentally confirmed that this solution could not compensate for the prolongation of the written mark length accurately and also it is practically impossible to change the power of writing laser beam pulses precisely. Furthermore, the difficulty (b) mentioned previously was not addressed at all, while (b) is the most important cause of RDL errors.